1. Field of the Invention
The invention relates to an internal combustion engine comprising a cylinder head with at least one intake port and at least one fuel injection device per cylinder, which extends into the combustion chamber. The invention also relates to a method for operating an internal combustion engine with at least two exhaust valves per cylinder, where in at least one operating region of the engine exhaust gas is recirculated into the combustion chamber from the exhaust ports via the exhaust valves, and where a swirl flow is generated in the combustion chamber due to differing closing times of the exhaust valves. Furthermore the invention relates to an internal combustion engine comprising a cylinder head with at least two intake and two exhaust valves per cylinder, and with a valve actuating device which permits closing of the exhaust valves at different times. The invention further relates to a cylinder head for an internal combustion engine with at least two intake ports per cylinder opening into the combustion chamber, each intake port being provided with a valve seat in the area where it opens into the combustion chamber, and the openings being partly surrounded by masks formed by material projections of the top face of the combustion chamber.
2. The Prior Art
From DE 103 26 054 A1 there is known an internal combustion engine with a cylinder head, an injection device injecting fuel directly into the combustion chamber, and two intake ports per cylinder, where the intake openings are provided with rib-shaped masks surrounding the disks of the intake valves semi-circularly and facing the injection nozzle. These masks deflect the combustion air flow approximately in parallel to the lateral fuel jet in order to initiate a tumble flow in the combustion chamber. In the area of the fuel jet at least one of the masks is divided by a gap, exposing the fuel jet immediately to part of the inflowing combustion air. This should increase the efficiency of the internal combustion engine and improve ignition reliability. The gap extending over the whole height of the mask will however impair the effect of the mask.
In conventional direct-injection internal combustion engines the injection device extends into the combustion chamber. The exposed situation of the injector tip causes increased wear.
From U.S. Pat. No. 6,502,541B2 there is known an internal combustion engine in which exhaust gas can be recirculated into the combustion chamber via the exhaust valves. Internal exhaust gas recirculation is used to improve fuel consumption especially in part-load operation. To generate a swirl of the recirculated exhaust gas in the combustion chamber the exhaust valves are closed at different times. Different closing times are achieved by a phase shift in the timing of the two exhaust valves. The exhaust valves are thus also opened at different times. In order to adjust the valve timing for both valves independently of each other complex technology is required.
In U.S. Pat. No. 5,870,993 another internal combustion engine is disclosed with two intake and two exhaust valves per cylinder, in which internal exhaust gas recirculation from the exhaust ports into the combustion chamber can be achieved by shifting the lifting curves of the exhaust valves. By masks in the area of the two exhaust ports a swirl may be imparted to the recirculated exhaust gas in the combustion chamber. The masking of the two exhaust ports is disadvantageous at full load.
From EP 0 764 770 B1 there is known a cylinder head which is partially provided with masks in the area where the intake ports open into the combustion chamber, wherein the walls of the intake ports on one side of the opening are configured in such a way that the flow cross-section at low lift of the intake valve is narrowed over an angle region of approximately 180°. At low-load or part-load operation this creates a tumble flow which arises from the intake ports.
U.S. Pat. No. 4,974,566A discloses an internal combustion engine with two intake ports opening into the cylinder, whose walls are configured such that at small valve lifts the intake cross-section for the flow through the intake ports is narrowed in a region defined by a certain angle around the movement axis of the intake valve in such a way that at small valve lifts a tumble flow is generated. For large valve lifts the intake cross-section extends along the whole periphery of the valves, which permits satisfactory filling of the cylinder. In the part-load region the engine is operated with small valve lifts, thus giving good thermodynamic conditions for combustion due to the prevailing tumble flow. At full load the engine is operated with full valve lifts, thus achieving sufficient filling of the cylinder and sufficient torque.
It is the object of the present invention to minimize the above mentioned wear of the injection device in an internal combustion engine. Furthermore, the forming of deposits in the area around the entry of the injection device into the combustion chamber is to be avoided. A further aim of the invention is to achieve good fuel economy at part load without impairing full load performance. Furthermore it is an object of the invention to design a cylinder head with which combustion conditions especially at part load may be improved and emissions may be further reduced.
The invention achieves these aims by providing that the top of the combustion chamber has an injector pocket in the area where the injection device enters the combustion chamber, preferably with at least one arrangement of scavenging passages opening into the injector pocket. Scavenging air enters the injector pocket via the scavenging passage arrangement, thus preventing the formation of deposits. The method of scavenging the injector will not, or only slightly, impair the effectiveness of the masking.
In a first variant of the invention, which has no negative influence on the effectiveness of the masking, it is provided that the scavenging passage arrangement is located in the region of at least one squish surface of the combustion chamber top preferably formed by the cylinder head. Preferentially it is provided that the scavenging passage arrangement has at least one scavenging passage which starts from a flat entry area and opens into the injector pocket via a nozzle region. Advantageously the depth of the scavenging passage increases in the direction towards the injector pocket. Preferably the scavenging passage narrows in the form of a nozzle, thus causing the scavenging air to enter the injector pocket with high flow velocity. As the piston approaches upper dead center scavenging air is pressed via the squish surface into the scavenging passage arrangement and further into the injector pocket, thus cleaning out deposits from the injector pocket.
In a second variant of the invention it is provided that the scavenging passage arrangement departs from an intake port, preferably from the valve seat area of the intake port. If at least one intake port is furnished with a mask, it is of particular advantage if the scavenging passage arrangement is formed into the mask. Advantageously in this case at least one scavenging passage is positioned essentially radially relative to the intake port between the intake port and the entry point of the injection device.
In a particularly simple variant of the invention it is provided that the scavenging passage has a cross-section which is open towards the combustion chamber and is configured preferably as a slot or a groove. The slot or groove may be machined into the cylinder head in a simple manner. It is also possible, however, that the scavenging passage at least partly has a closed cross-section and is preferably formed by a bore. Preferentially it is provided that the scavenging passage arrangement has a certain distance from the bottom of the mask. The effective mechanism of the mask will be much less impaired by this measure than by a gap in the mask extending over its whole height, as shown in DE 103 26 054 A1.
It is provided by the invention that the longitudinal axis of at least one scavenging passage forms an angle greater than 0°, preferably between 30° and 60°, with a plane defined by the cylinder axis and the axis of the injection device.
To achieve a sufficient swirl effect it is advantageous if the mask extends around the intake port over an angle of 150° to 180°. In this instance the height of the mask should be between 1.5 mm and 4 mm. Preferably it is provided that the distance from the mask to the rim of the valve disk is about 0.3 mm to 0.7 mm.
To improve on fuel consumption at part load without impairing full-load performance it is proposed that the two exhaust valves be opened for differing lengths of time, both exhaust valves preferably opening at the same time. Via a phase adjuster the opening point of the exhaust valves may be shifted synchronously. In the case of cam-controlled operation in particular, it may be provided that the exhaust valves have exhaust valve lift curves of differing length.
The invention is also suitable for cam-less operation, however.
By simultaneously opening both exhaust valves a relatively large volume of exhaust gas may be fed into the exhaust ports at the beginning of the exhaust stroke. In this way a favourable emptying behaviour of the combustion chamber may be achieved with minimal throttling losses. This will result in a high power yield, especially at full load.
In the context of the invention it is provided that the difference between the closing points of the exhaust valves is 10° to 80° of crank angle, and preferably 20° to 60° of crank angle, a first exhaust valve preferably being closed immediately after upper dead center of the charge exchange process and a second exhaust valve being closed at 20° to 60° crank angle after upper dead center of the charge exchange process. The closing point is here defined as that point in time at which the exhaust valve has a residual lift of 1 mm. Differing closing points will cause swirl to be generated when exhaust gas is sucked back into the cylinder from the exhaust duct. To achieve strong swirl components it is advantageous if the recirculated exhaust gas is guided over a flow guiding surface formed by a mask in the area of at least one exhaust valve. The mask may be provided in the area of one exhaust valve only. In this way flow losses during outflow from the cylinder at full load may be kept small. In order to achieve a sufficient swirl effect it is advantageous if the mask extends around the center of the exhaust port through an angle of approximately 150° to 180°. The height of the mask should be 1.5 mm to 4 mm. Preferably it is provided that the distance of the mask from the rim of the valve disk is 0.3 mm to 0.7 mm approximately.
The different closing times of the exhaust valves may be realised by an asymmetrical shape of the cam of at least one exhaust valve. Preferentially it is provided that each exhaust valve is actuated by its own cam, with the cams having different closing flanks. Furthermore it may be provided that the cams have identical opening flanks and/or identical maximal cam lobes.
In addition, the swirl in the combustion chamber due to the intake flow from the intake ports can be increased by asymmetrical intake ports, one intake port being preferably configured such that it can be closed down.
Combustion conditions and emissions can be improved if the contour of the mask of at least one intake port has a flat main part of maximum height between an ascending and a descending flank and in a developed view is of asymmetrical shape, ascending and descending flanks preferably having different slope angles.
It is particularly advantageous if the flank of the profile closer to the combustion chamber wall has a smaller slope than the flank nearer to the center of the combustion chamber. The intake flow may thus be specifically guided to a central cylinder region generating a tumble flow in the direction of the piston axis.
It has been found by experiments that a particularly high stability of combustion can be attained if the mask of each port extends over an angle of 120° to 210°, measured around the center of the port opening, and preferably over an angle between 160° and 180°. Preferentially it is provided that at each port a main axis running approximately through the middle of the main part of the mask forms an angle of 70° to 120°, preferably between 80° and 110°, with a straight reference line running through the centers of the port openings.
The height of the mask is chosen such that at partial valve lift the opening of each intake port is laterally covered on the intake side. At full valve lift the intake valve is beyond the mask and the full intake cross-section is available. The height of the mask is 1.2 mm to 3.5 mm approximately, and preferably 1.6 mm to 2.5 mm, and even more preferably 1.6 mm to 2.2 mm. In order to avoid heavy throttling of the intake flow especially under full load it is of advantage if a clearance is provided between the mask and the valve disk, which should not exceed a quarter of the height of the mask, preferably.
In a particularly preferable variant of the invention it is provided that the depth of the injector recess is at most about equal to the height of the mask. By positioning the injector orifice in the area of the injector recess of the mask, the injector nozzle may extend relatively deeply into the combustion chamber. The depth of the injector recess will thus correspond to the height of the mask at most. It is furthermore of advantage if the injector recess is at a distance from the wall of the mask, this distance being at least 1 mm. This minimum distance between injector recess and wall of the mask will avoid negative influence on the intake flow.
The invention will now be described in more detail with reference to the enclosed drawings.